In his U.S. Pat. No. 4,022,380, the applicant discloses a fluid flow control valve comprising a casing, an inlet to the casing, an outlet from the casing, a chamber having a movable wall part and a fixed wall part, a restricted inlet to the chamber through one of said wall parts, said restricted inlet communicating with the inlet to the casing. An elongate cleaning element is provided for clearing dirt and scale from said restricted inlet. A valve element normally closes an outlet from said chamber. Actuating means are provided for displacing both said cleaning element and said valve element whereby said cleaning element moves in said restricted inlet to the chamber and said outlet from the chamber is opened to permit liquid to flow from said chamber and allow said movable wall part to move in the direction which reduces the volume of said chamber. A valve seat and co-operating valve surface for controlling flow from said inlet of the casing to said outlet of the casing are arranged to separate upon said movable wall part moving in said direction.
In a preferred form, the movable wall part comprises a frusto-conical diaphragm exposed internally to pressure within the chamber and externally to pressure in the casing, a plate within the chamber, said plate being secured to said diaphragm at a central region of the diaphragm and having a frusto-conical shape commensurate with the shape that the diaphragm adopts while the valve outlet is closed so that, while the valve outlet is closed, the plate is in face-to-face contact with the inner surface of the diaphragm over substantially the whole area of the diaphragm, said diaphragm having freedom to move from said position it adopts when the valve is closed to a position in which the conical angle is essentially reversed.
Experiments have shown that, when the valve is closed, the plate engages and supports virtually the entire area of the diaphragm. In these conditions it can be considered to be a rigid wall. As the valve opens, the plate and disphragm separate, the conical angle of the plate remaining the same and the conical angle of the diaphragm reducing. During closing the plate and diaphragm return to their face-to-face condition. During the initial closing movement it is only the central region of the diaphragm and plate which are in engagement. The diameter over which the plate and diaphragm are in engagement progressively increases during closing and, at closing, there is full face-to-face contact. Applicant's experiments have shown that water between the plate and diaphragm must be forced out of this region through the annular gap which exists between the periphery of the plate and the diaphragm. This gap is narrow and a `squeezing` action must be exerted on the water to expel it. This in itself leads to some resistance to closing with an immediate reduction in the possibility of water hammer being created. Another important factor in resisting the creation of water hammer is that as the valve approaches its closed condition virtually the entire area of the diaphragm is supported. It consequently cannot flex inwardly should the central portion suddenly be sucked into its seat. Thus although there is certainly a tendency to suck the valve onto its seat this must of necessity lead to an increase in the volume of the chamber. As the diaphragm is prevented from flexing inwardly to compensate for the tendency of the chamber to increase in volume, an internal suction effect is created within the chamber whcih prevents the valve snapping shut under the suction effect of the flowing water.
In initial production samples, rigid metal cleaning elements carried by stems fixed to the valve element were used to clean the restricted inlet to the chamber. These elements were normally outside the opening and moved into and through the opening in use. In order to ensure that the valves seated accurately and that the movable wall part moved without undue interference, distortion or twisting, it was found to be desirable accurately to align the cleaning element with the restricted inlet, with the need for increased manufacturing accuracy. A misaligned rigid cleaning element could enter the restricted inlet and twist or distort the movable part slightly, particularly when a diaphragm and plate were used. Then, apart from slight sticking of the element in the restricted inlet, the element could be rapidly returned to its misaligned state by resilience in the movable wall part on leaving the restricted inlet with a resulting rapid movement of the movable wall part. This, together with the increase in flow area when the element leaves the restricted inlet, could lead to slight water hammer. The metal elements can also scratch and gouge the interior of the opening in many cases, resulting in inaccurate metering.
It is an object of the invention to further improve fluid flow control valves from a point of view of reducing both water hammer and the degree of accuracy required in manufacture.